Such devices are known also, for instance, by the technical term "equipment protection devices". Typical fields of application for equipment protection devices are electronic data processing installations, especially individual components thereof, and similar electronic equipment, such as measuring, regulating and control units, switching and exchange apparatus, CNC production machines and industrial robots, CAD/CAM systems, as well as printers. It is likewise known that the electronic modules of such instruments or machines must be cooled, for instance by ventilation, because of the heat they develop, a distinction being made, in accordance with the generation of the ventilation, between forced ventilation type apparatus, on the one hand, with which the cooling air stream is generated by a fan inside the apparatus and naturally ventilated apparatus, on the other hand, with which convection of the room air, either natural or produced by air conditioning apparatus, is utilized by virtue of the particular arrangement of the equipment at the place of installation. The term "fire characteristic magnitude" is used to designate physical magnitudes underlying measurable alterations in the surroundings of a nascent fire, such as the ambient temperature, the contents of solids, liquid, or gas in the ambient air (formation of smoke--particles or aerosols--or vapor) or ambient radiation.
The significance of fire detecting devices or in short: equipment protection devices is on a constant rise in parallel with the greatly increasing dependence on electronic data processing or electronically controlled manufacturing processes in companies of all kinds. Whereas fire protection measures, until some years ago, still were tailored to preserving the buildings, nowadays fires must be detected as early and as reliably as possible at the appliances or machines themselves in order to recognize a fire in its originating phase already. The shortest possible time span between the moment of origination of a fire and the moment of detection of the fire, as well as the corresponding measures, is of the greatest importance, for example, with the electronic equipment mentioned intially, especially because with such equipment it is not the primary damage to the particular appliance which is decisive but instead the secondary damage caused by strong development of smoke in the affected room. After all, the material subject to burning above all is plastics, like PVC and polyethylene, such as used in cable insulations which release hydrochloric acid gases upon combustion, reacting with water withdrawn from the humidity of the air to form hydrochloric acid. The latter then deposits as very fine fog on the appliances or machines in the room and also enters into them via the room air. As a consequence corrosion processes take place whose repair often leads to shut-down of an entire plant.
The difficulty with early detection of fires in ventilated appliances or machines or, generally, in the rooms in which such appliances are installed lies in the circulation of the air generated by ventilation of the appliances or of the room, which circulation is intended with the aim of obtaining the best possible cooling. In air conditioned rooms, for example in computer centers, the currents of air which predominantly are directed from the bottom to the top often do not reach the ceiling of the room so that smoke contained in the air stream can be detected only very late by the known spot alarms frequently installed at the ceiling. Another aspect is that the main current of cooling air in a modular type of apparatus, for instance, may change due to the fact that first the apparatus is being used with less slide-in units and, subsequently, the addition of further slide-in units will considerably alter the distribution of cooling air. As a result, the placing of the conventional spot alarms later on often is no longer correct.
It was in recognition of this fact that the fire detecting devices mentioned initially were developed whose measuring chamber is put directly on the apparatus to be monitored. These known fire detecting devices, for example, suck a fraction of the main cooling air stream through a fan and deliver this cooling air stream to the detectors located in the measuring chamber. To be able to detect the main current of cooling air, these known fire detecting devices aspire the partial air by means of funnels or suction tubes placed on the air outlet openings of the ventilated appliance or machine. In this context it must be observed that regulations stipulate that the cooling air stream of the corresponding appliances not be impaired.
The problem with these known fire detecting devices is that due to the constant change in apparatus technology the masses of air flow and also the flow velocities steadily increase so that it becomes ever more difficult to tap a fraction off the main current of cooling air of the ventilated appliance and permit reliable detection of a fire characteristic magnitude under good measuring conditions, especially the fire characteristic magnitude "proportion of solids or liquid in the air". Frequently the situation is such that a strong vortex forms in the cooling air under the funnels or in the measuring chamber so that it would take too long to detect, for instance, solid particles in the cooling air (smoke). The detection times, when using the known fire detecting devices of the kind mentioned initially, are approximately 60 seconds which still is too long.